In Sequence Color and Memory Television Systems, hereinafter referred to as SECAM, the transmitted color subcarrier alternates between two color difference signals from line to line. For this reason SECAM color difference signals, D'.sub.B and D'.sub.R, alternately modulate the subcarrier. As a result of this frequency modulation, the color signal is less sensitive to differential phase and differential gain. However, since only one color difference signal is transmitted at one particular time, some memory device must be used so that such color difference signals are available simultaneously, say, in the receiver or image producing device such as a color picture tube. Herein then, lies a disadvantage of the prior art, namely non-ideal delay line.
As is well-known, the last color information to enter the SECAM delay line prior to the viewed line was the opposite color difference signal, any reflections due to such non-ideal delay line termination or construction will appear as cross colors at the output of the delay line. This cross color may be present from multiple reflections, with its amplitude reflecting the amount of time it has been present in the delay line. Some direct transmission through the delay line may also be present, but this is principally a property of the delay line construction. Such cross color represents a deterioration of a theoretical advantage over other systems of color television transmission which is basic to SECAM.
A second disadvantage is that in steering the alternating lines of color information from the output of the delay line and direct transmission to the input of the D'.sub.R and D'.sub.B demodulators, some cross talk must occur in the switch. Where the signals transmitted are analog, this represents a noticeable degradation of the chrominace signals.
Following the adding together of the previously mentioned color difference signals, such signals must be amplitude limited and frequency detected due to frequency modulation as discussed. The frequency detector, or modulator as hereinafter referred, produces an output dependent upon how much an input signal differs in frequency from an undiviated or rest frequency. In other words, amplitude variations of the color difference signals are derived in response to frequency variations. Thus, another disadvantage of the prior art.
As is well-known by those skilled in the art, many means of demodulation of a frequency modulated signal are known, one of which is the phase lock loop. In SECAM color systems large frequency deviations of the color subcarrier occur at a very fast rate. Because of this, it is very difficult to build a phase lock loop demodulator. Inherent in the construction of such demodulator is high loop gain and large loop bandwidth which tend to decrease the advantage of a phase lock loop demodulator over known methods of frequency detection.
In an article written by C. J. Byrne entitled "Properties and Design of the Phase Controlled Oscillator with a Sawtooth Comparator" and published in the Bell System Technical Journal, March 1962, means including a sawtooth phase comparator are discussed to overcome the disadvantages of the more common sinusoidal phase comparators and thereby construct a phase locked loop which would be improved in some respects over phase locked loops mentioned above. Such improvement, if carried further, could be used to make a more improved phase lock loop demodulator for a SECAM color system.
The disadvantages of the prior art were overcome by the invention described in U.S. Pat. No. 3,863,264 in that the color difference signals are digitized at the incoming subcarrier level prior to being applied to the delay line. Digitizing of such signals eliminates the effect of cross talk within the switch. Further, routing of the color difference signals through multiple delay lines reduce reflections below any desired level. These two advantages combine to provide virtually no cross color, a theoretical advantage of the SECAM system. A further advantage is that the digitizing of the color difference signals at the incoming subcarrier level provides better equivalent noise bandwidth because of the digital phase detector which enables a lower bandwidth phase lock loop to be used.
The invention described in U.S. Pat. No. 3,863,264 further overcame the disadvantage of the prior art in that any switching before or after the delay line(s) can consist of simple logic gates. Also, by using digital signals corresponding to the color difference signal subcarrier and decoding such signals based upon both a positive and a negative transition, a very appreciable increase in decoder accuracy, speed, and equivalent noise bandwidth can be obtained.
As is well known, the trigger points of the Schmitt trigger used for the waveform squaring circuit described in U.S. Pat. No. 3,863,264 must be chosen at a sufficiently high level to ensure enough delay between the modulator-demodulator transition and the Schmitt trigger transistion to obtain a clock pulse of usable width. To produce a usable width clock pulse for the highest level input signal which can be expected, the trip points of the Schmitt trigger must be selected at an amplitude, say 10 percent of the expected input peak value. This sets the lowest level input signal for which the waveform squaring circuit will produce an output. A disadvantage, however, is that frequently the level selected is not low enough for reasonable fluctuations in input signal level.